1. Field of Invention
The present invention relates to a photosensitive composition used in lithography and, more particularly, to a photosensitive composition having an improved oxygen-plasma resistance.
2. Description of the Related Art
Lithography is an essential micro-patterning technique in the manufacture of such semiconductor devices as LSIs. As a result of the recent trend toward highly-integrating elements in an LSI, the elements are now not only micro-patterned but are also arranged three-dimensionally as in multilayer interconnection. However, as a consequence of three-dimensional arrangement having become adopted in semiconductor devices, the following new problem has arisen with regard to lithography.
In order to manufacture a three-dimensionally integrated device by use of lithography, a resist pattern must be formed on an underlying surface having a relatively large step. In order to accurately transfer a pattern of a mask onto a resist film which is formed on the underlying layer having a large step by exposure to light, the thickness of the resist film must be increased in order to ensure a flat resist film surface. However, an increase in the thickness of a resist film results in a decrease in resolution, this being for a variety of reasons.
One way of solving the above problem is to use a multilayer resist system (Solid State Technology, 74, 1981). The conventional multilayer resist system is a three-layer resist system in which three resist layers are employed, stacked one upon another. The function of the lowermost layer is to planarize the steps on the underlying surface and prevent light from reflecting therefrom, which would cause a reduction in resolution. The interlayer serves as an etching mask when the lowermost layer is patterned. A resist having excellent etching resistance, especially with respect to reactive ion etching using oxygen plasma (hereinafter referred to as oxygen-RIE resistance) is used as the interlayer. The uppermost layer serves as a photosensitive layer. According to this method, after the uppermost layer is exposed and developed, the interlayer is patterned, using the uppermost layer pattern as an etching mask, after which the lowermost layer is patterned, using the interlayer pattern as an etching mask.
As can be seen from the above description, each layer of the three-layer resist system performs a specific function such as one flattening an underlying surface and preventing light from reflecting therefrom, another providing the oxygen-RIE resistance, and another providing the required photosensitivity. According to this method, a finer resist pattern can be formed with high accuracy on an underlying surface having a step, as compared with a conventional monolayer resist system. However, the three-layer resist system requires that dry etching be repeated several times to form a pattern, with the result that the manufacturing process becomes undesirably complicated.
For this reason, a two-layer resist system has been developed (J. Vac. Sci. Technol. B3,306, 1985). In the two-layer resist system, a single layer serves as the uppermost layer and the interlayer in the three-layer resist system, thereby simplifying an etching process. This system makes use of a photosensitive silicon-containing polymer which has both satisfactory photosensitivity and oxygen-RIE resistance. Conventionally known examples of such a silicon-containing polymer are chloromethylated polysiloxane (Japanese Patent Disclosure No. 1985-59347), which has a chloromethylated benzene ring directly bonded to a silicon atom, and silicon-modified novolak resin (Japanese Patent Disclosure No. 1986-198151).
However, the above conventional two-layer resist system has the following drawbacks:
A conventional photosensitive composition comprising a silicon-containing polymer which is polymerized by radiation beams or lights requires the use of an organic solvent as a developing agent. For this reason, a pattern formed by the development process absorbs the organic solvent and swells thereby reducing the pattern accuracy.
In addition, novolak resin, in which a silicon atom is directly bonded to a benzene ring of a phenol-derivative residue, is very expensive because the yield of silicon-modified phenol, which is a monomer for the novolak resin, is extremely low.